In a case to perform LED illumination with commercial AC power, full-wave rectification is performed with a diode bridge circuit on output of a dimmer connected to the AC power, and an LED is operated via a transformer (flyback convertor) having a flyback configuration. At that time, an LED current control apparatus controls a current flowing through a primary winding of the flyback convertor with a switch circuit to control a current flowing through a secondary winding. A conventional LED current control apparatus monitors a width of a current flowing through the secondary winding, and PWM control is performed on switching timing of the switch circuit connected to the primary winding, for example. Thus, the LED connected to the secondary winding is lighted with mean current control.
When peaks of the AC input voltage are constant, energy accumulated at the primary winding of the flyback convertor is constant as well. Accordingly, energy supplied to the LED connected to the secondary winding is constant and brightness of the LED illumination is stabilized as well.
However, peak voltages of commercial AC powers largely vary in accordance with regions and infrastructures to be in a range of 100 to 240 V as standard. With a conventional LED current control apparatus, there has been a problem that a mean LED current fluctuates in proportion to magnitude of a peak voltage of an AC power and that brightness of LED illumination fluctuates as well.
Further, since an LED has smaller consumption power than a conventional filament lamp and a phase of an AC input current waveform is not delayed owing to that an LED is not an induction load, there has been a problem that a high power factor is difficult to be obtained.